Humoral hypercalcemia of malignancy is a common paraneoplastic syndrome which remains a serious therapeutic problem. Its pathogenesis is poorly understood. Identification of the responsible mediator or mediators in this syndrome had been an area of considerable investigative interest. We have recently purified and sequenced a peptide from two human and two animal tumors which shows intense N terminal sequence similarity with parathyroid hormone but differs in the rest of its primary structure (hACSP). Full length cDNA clones for this molecule have been identified and synthetic 1-36 and 1-74 fragments of the full length molecule synthesized. We have shown that synthetic 1-36 hACSP and 1-74 hACSP are potent bone-resorbing agents in vivo and in vitro. We have also isolated a similar factor from normal human keratinocytes and identified mRNA species for this molecule in several normal tissues. Taken together, these data suggest that this peptide is a candidate for the mediator of HHM and is secreted by non-malignant cells as well. In studying the effects of this molecule on bone, we have demonstrated that osteoblast-like cells when treated with hACSP release bone-resorbing activity. We have partially purified this bone-resorbing activity and shown that it has a molecular weight of less than 6500 kd and is not due to prostaglandin E2. We have also demonstrated that these osteoblast-like cells secrete a T cell growth factor in response to hACSP which has biochemical and immunological properties consistent with those of granulocyte macrophage colony-stimulating factor. The goals of the current proposal are to: 1) Further characterize the nature of the bone-resorbing activity released by clonal and primary osteoblast-like bone cell cultures in response to hACSP. 2) To study the nature and spectrum of released cytokines by osteoblasts in culture in response to hACSP. In these studies we plan to use both less specific functional assays such as rat marrow culture systems as well as cytokine-specific assays such as the HT2 assay. 3) Analyze the effects of these peptides on osteoblast replication and differentiated osteoblast function such as collagen production, osteocalcin release, and alkaline phosphatase activity. We expect that these studies will help illuminate the pathogenesis of the syndrome of humoral hypercalcemia of malignancy and may begin to define a physiologic role for this newly identified hormone.